The present invention relates to a sand blasting nozzle having a flat jet and containing solid abrasive particles. It more particularly relates to a sand blasting nozzle using water under high or very high pressure for cleaning very dirty surfaces, or surfaces covered with oxides, paint or various deposits. It also relates to a process for using a sand blasting nozzle for radioactive decontamination.
During the operation of nuclear power stations and units for the reprocessing of nuclear fuels, certain elements are exposed to radiation. A thin radioactive film forms on the surface of these elements and this film must be eliminated. This cannot be obtained by using pressurized water alone, because the latter does not make it possible to eliminate oxides. It is therefore conventional practice to use sand blasting devices, which employ high pressure water in which the water jet contains abrasive particles. A sand blasting device of this type comprises an intake device, which produces a jet within a vacuum chamber. A pipe, which supplies a mixture of air and abrasive, also issues into said chamber. The vacuum makes it possible to suck in the abrasive, which is then incorporated into the water of the jet.
Known sand blasting devices of this type are constructed in two different ways.
In the first type, the water is injected along the axis of the sand blasting device and the abrasive supply pipe issues laterally into the vacuum chamber. This construction permits reduced overall dimensions, a simple design and a large concentration of the water jet. However, a disadvantage thereof is that all the mixed air -solid-water jet has an abrasive effect on the discharge nozzle and in order to limit these effects, the nozzle must be made from a mass of a very hard material, such as carbide.
According to a second possible construction, the abrasive particle supply pipe is arranged in the axis of the device and one or more discharge nozzles are positioned laterally with respect to said pipe. Three, four or six nozzles with cylindrical discharge tubes are arranged in annular manner and converge at a point positioned close to the sander outlet. This arrangement offers the advantage of only a slight exposure of the discharge nozzle walls to the action of the abrasive particles, because the mixed jet is not homogeneous. Thus, the particles are essentially concentrated in the center of the jet. Moreover, the impact of the jet on the member to be cleaned is distributed over a larger surface.
However, no matter what constructional mode is envisaged, the known sand blasting devices have a circular path on the member to be cleaned, when its surface is presented perpendicular to the jet and semi-elliptical when the surface is presented at an angle of incidence below 90.degree.. In the latter case, the ejected abrasives have a degressive action which is inversely proportional to the distance between the sander and the treated object and erosion will be greater at the apex of the semi-ellipse.